Process control plants or systems often employ rotary valves, such as ball valves, butterfly valves, etc., to control the flow of process fluids. In general, rotary valves typically include a fluid flow control element or member that is disposed in the fluid path and rotably coupled to the body of the valve via one or more shafts. Typically, a portion of a shaft extends from the valve to function as a valve stem, which may be operatively coupled to an actuator (e.g., a pneumatic actuator, an electric actuator, a hydraulic actuator, etc.)
In operation, a controller may cause the actuator to rotate the valve stem and, thus, the control member to a desired angular position to vary an amount of fluid flowing through the valve. When the valve is closed, the control member is typically configured to engage an annular or circumferential seal that encircles the flow path through the valve to prevent the flow of fluid (e.g., in one or both directions) through the valve.
The control element or member within a ball-type fluid valve, which is commonly referred to as a ball valve, typically has a generally spherically shaped or otherwise curved sealing surface that is configured to engage a circumferential sealing ring. Some ball valves utilize a flow control member having a spherically shaped sealing surface with a substantially constant or single spherical radius of curvature. Although ball valve control members having a single radius of curvature are relatively easy to manufacture using automated processes (e.g., using a computer numerical control (CNC) machine), such single radius control members have some operational drawbacks. For instance, ball valve control members having a sealing surface with a single radius of curvature require a mating sealing ring to fully deflect upon contact with the leading edge of the control member and through a relatively large angle of engagement, which can prematurely wear or otherwise damage the sealing ring.
Other ball valve designs, such as that disclosed in U.S. Pat. No. 3,456,916, employ a contoured control member surface having a variable radius of curvature. The variable radius of curvature serves to gradually deflect the sealing ring when the control member engages the sealing ring during, for example, closing of the valve, thereby increasing the cycle life of the sealing ring. However, the manufacture of such a variable radius ball valve control member typically requires costly, error prone manual grinding operations to smooth or blend the substantially different spherical radii that are normally used. For example, such grinding operations may be needed to eliminate a ridge or the like formed at the intersection of the different curvature radii. Unfortunately, during such manual grinding operations it is difficult to avoid compromising (e.g., by over grinding) the primary spherical radius of the sealing surface (i.e., the portion of the sealing surface that engages with the sealing ring to prevent the flow of fluid through the valve). Thus, it would be desirable to provide a ball valve control member having a contoured sealing surface that does not require additional manual blending or smoothing operations to eliminate ridges and/or other non-seamless types of transitions between sealing surface regions having different curvatures.